One gleaming skyscraper has a sideways restaurant, where people sit at a ninety-degree angle to the ground eating seafood while the updraughts from street-sweepers stir their hair. They appear to be living in a different gravity from the rest of us. I seem to remember being told that to get up there you have to offer certain bribes to the concierge, but I never asked what.

(The concierge, by the way, is a tulip.)

Sometimes there are snorting horses drawing carriages over cobbles, and steaming reeks rise from manhole covers as if an underground machine is gearing itself up for a very rude belch. Sometimes the buildings are square and shiny plastic, as if they’ve drifted in from the archetypal sleeping Lego Republic.

Sometimes the city is muted to utter silence.

Sometimes the streets are metal grates over jungles that release a fume of fruit and rot and animal smells. You can’t get down there or see beyond the top of the canopy, but you can taste the moisture in the air of all that trapped life and you can hear the birds and insects in full cry. Once I saw a tiny drone fly down between the bars and amongst the leaves, and there was a crunch of metal being snapped and the sound of a motor wailing and then cutting off, and the birds went silent, just for that moment, before beginning to shriek.

One of the city’s more curious features is the way you forget most of it on waking, yet when you return in a future dream, you find you can recall everything that ever happened here in vivid detail. Sometimes these memories unfold in several layers like an accordion fan; you see a certain doorway or staircase and it’s like you clicked a hyperlink that offers up its long backstory in your own dream history – or maybe (I suspect) someone else’s.

You never know what you will find around the next bend. Once I was in someone’s flat and stepped out onto a balcony only to be confronted by a vast plain of water, apparently without waves, stretching out for ever, in all directions. It appeared to surround the building; yet when I went back inside the flat and looked out of a different window, I saw only a multistorey car park complete with two drunks having a pissing contest against the wall.

This time I’m picking my way along the bank of the swollen river, one hand on a slimy chain-link fence that separates the crumbling bank from the frontage of buildings. Sodium lamplight turns the droplets that collect on the links to gold, and they shower down on me every time I grab the fence for balance. On the landward side there are Plexiglas flood defences, but these have big holes in them as if giants have been chucking rocks at each other. Up ahead I can see a place where someone has taken wire-cutters to the fence and ripped back a panel.